1. Field of the Invention
This invention relates to the production of hydrogen and carbon monoxide by reacting a source of carbon with steam at a high temperature. More particularly, this invention relates to the production of hydrogen and carbon monoxide by reacting coal with steam wherein the heat of reaction is provided substantially completely by the steam reactant.
The increase in cost and short supply of petroleum in recent years has made coal gasification a viable alternative for providing fuel. One of the problems associated with the use of coal itself as a fuel is that much of the coal mined contains a high amount of sulfur. Sulfur oxide pollution control regulations prohibit the emission of more than 1.2 pounds of sulfur dioxide per million BTU's of heat generated by the burning of coal. In coal gasification, however, the sulfur can be easily removed from the product gas so that a plant utilizing the water gas as a source of fuel will not ordinarily require expensive and sophisticated pollution control apparatus to meet regulations.
An economically feasible coal gasification plant for the purpose of supplying fuel to industrial and utility consumers can (1) provide enough fuel to make the United States energy self-sufficient, (2) satisfy air pollution control regulations, and (3) revitalize the coal mining industry in the high-sulfur bituminous coal districts.
2. Prior Art
The "water gas" reaction is the reaction of a carbonaceous solid with steam at high temperature to produce hydrogen and carbon monoxide and has been practiced for many years. The water gas reaction is endothermic, requiring a substantial amount of heat for reaction. One method currently used for providing the heat of reaction is to provide fuel and oxygen for combustion within a gasification reactor. A number of difficulties are encountered with this method. The hot combustion gases, principally CO.sub.2, are entrained with the product gas, in addition to any gases which may accompany the oxygen, such as nitrogen. The product gases are diluted with these undesirable entrained gases, causing considerable difficulty and expense in recovering the hydrogen and carbon monoxide. Further, sulfur and other impurities may accompany the carbonaceous solid and are oxidized in the combustion reaction to sulfur oxides and other undesirable gaseous products of combustion. These oxides must also be separated from the product gas.
One attempt to solve the problem of dilution of the water gas with undesirable entrained gases is disclosed in the Seglin et al., U.S. Pat. No. 3,787,193. As set forth in the Seglin et al. patent, molten slag can be provided in the gasification reactor to supply the necessary heat for the water gas reaction, thereby eliminating the need for combustion within the gasification reactor. In the Seglin et al. process, a separate heating section is provided for producing the molten slag so that the slag can be conveyed to the gasification reactor substantially free of oxidizing gas. In this manner, combustion gases are not entrained with the products of the water gas reaction.
Another method of preventing the dilution of the product gas with the combustion products is by indirectly heating the carbonaceous solid in a fluidized bed heat exchanger so that the hot combustion gases are not intermingled with the fluidized bed of carbon. One form of this process is disclosed in the Atwell, U.S. Pat. No. 2,680,065.